1. Field of the Invention
This invention relates to micro-fabricated actuators, and more particularly relates to improving long-stroke deflection characteristics.
2. Description of the Related Art
The advent of micromachining has enabled the economic fabrication of tiny precision micro-actuators and micromachines using techniques first pioneered in the semiconductor industry. Micro-fabricated actuators with long stroke are used in a diverse range of applications including adaptive optics, disk drives, fluidic valves, video displays, and micro-positioning.
Microfabricated actuators are often comprised of an actuation means acting on a body or platform mounted to a substrate via a flexible suspension. The suspension allows the actuator to move while providing a restoring force that is a function of deflection. The restoring force allows precise actuator positioning at equilibrium points where the restoring force counter balances the applied actuation force. The design requirements to ensure good deflection characteristics for the actuator are manifold. The suspension must be rigid enough so actuator natural frequency is above the minimum needed for fast dynamic response. In addition, the suspension must have enough rigidity to ensure robust mechanical shock and vibration survival. On the other hand, the suspension must be flexible to allow full scale deflection below the maximum actuation force. As the actuator is deflected, the suspension should not warp the body nor cause any excessive extraneous motion that is not in the desired direction of actuation. Finally, the suspension must be as compact as possible to fit within a small footprint to reduce device area and hence cost. A compact suspension is even more critical for tightly packed arrays of actuators such as optical cross connects or deformable mirror arrays.
Some devices require that a micromachined actuator move substantially perpendicularly to the substrate in a piston motion or move in a piston motion as well as rotating about the axes substantially parallel to the substrate in a tip/tilt fashion. Several designs have been invented in an attempt to provide acceptable deflection characteristics for such devices, however all previous solutions have serious drawbacks.
One field where micro-actuators are prevalent is adaptive optics. Adaptive optics (“AO”) refers to optical systems that adapt to compensate for disadvantageous optical effects introduced by a medium between an object and an image formed of that object. Horace W. Babcock proposed the concept of adaptive optics in 1953, in the context of mirrors capable of being selectively deformed to correct an aberrated wavefront. As shown in the prior art FIG. 1, a typical application adjusts the wavefront of incoming light 105 using a deformable mirror 100 formed by an array of actuators so that the outgoing light wavefront 110 has reduced aberrations. Numerous actuators in the form of mirrors are tightly packed to form a deformable mirror surface that locally alters light path length. The full system to correct light wavefront aberration is shown in FIG. 2. The light to be corrected 200 enters the device 205, reflects off the deformable mirror 210 and is divided using a beam splitter 220. One portion of the split light enters a wavefront sensor 230 that detects aberrations. A wavefront reconstructor 235 and mirror controller are used to shape the deformable mirror to remove light wavefront aberrations. The second portion of light from the beam splitter 220 enters the science camera 225. The correction performed using the deformable mirror improves the image resolution of the science camera. See John W. Hardy, Adaptive optics for astronomical telescopes, Oxford series in optical and imaging sciences 16, Oxford University Press, New York, 1998. Adaptive optics has a wide range of uses including correcting telescopes for atmosphere turbulence, correcting ophthalmic images for eye cornea distortions, and focusing laser.
Helmbrecht, in Micromirror Arrays for Adaptive Optics, PhD. Thesis, University of California, Berkeley (2002), discloses a segmented deformable mirror for use in AO applications that exhibits high fill-factor and offers the potential for high mirror stroke.
Early MEMS resonators and actuators, for example those pictured in U.S. Pat. No. 5,025,346 Tang (1991), attempted to achieve good deflection characteristics over large motions by using folded beam structures exhibiting strain relief. In U.S. Pat. No. 6,091,050, Carr disclosed a similar folded beam technique using two long bimorph flexures connected at one end forming a U-shaped suspension. The first bimorph is anchored to the substrate with the other end attached to the second bimorph. The second bimorph folds back parallel to the first bimorph and attaches to an actuator body. However, the folded suspension as documented has considerable limitations that make it impractical in practice.
In summary, the prior art does not provide good deflection characteristics for actuators moving substantially perpendicular to a substrate.